The present invention relates to a toner image transfer method and apparatus for electrostatic photography which enables a large number of copies to be produced from a single electrostatic image through repeated development and transfer.
In a known electrostatic copying apparatus, an electrically conductive grounded drum is formed with a photoconductive dielectric layer on its periphery. The drum is charged with an electric potential and is irradiated with a light image which causes conduction in the bright image areas to dissipate the electrostatic charge and form an electrostatic image. This image is developed by applying a toner substance to the drum, and the toner is transferred to a copy sheet and thermally fixed thereto.
As long as the interior of the copying apparatus is shielded from light except for an imaging exposure, an electrostatic image formed thereby will be stable for a long time. The process thereby has the potential of producing a large number of copies from a single electrostatic image through repeated development and transfer. Imaging the drum only once for a number of copies greatly increases the copying speed.
However, known copying machines are only able to produce a few usable copies from a single electrostatic image since the electrostatic image is degraded during the transfer step.
In order to facilitate transfer of the toner image from the drum to the copy sheet, it is desirable to provide a transfer roller or belt which presses the copy sheet between itself and the drum for toner transfer. The belt is electrically conductive and is empressed with an electric potential having the same polarity as the electrostatic image on the drum to attract the toner from the drum to the copy sheet. The electric potential is made as large as is practical in order to effect complete transfer. However, a problem exists in known copying apparatus in that charge transfer occurs between the belt and the drum during the image transfer step which degrades the electrostatic image on the drum to the extent that only a few copies may be produced from a single electrostatic image. Since the copy sheets are made of paper comprising many entangled fibers of a dielectric material with many interstitial spaces between the fibers, the electric charge on the belt is partially transferred to the drum through dielectric breakdown through the copy sheet to degrade the electrostatic image.
It is further known in the art to ground the belt and provide a dielectric layer on the periphery thereof which is electrified by corona discharge. This expedient has not overcome the charge transfer phenomemon. If the potential on the belt is reduced to prevent charge transfer, the result is insufficient transfer of the toner image. If the pressure between the copy sheet and the drum is increased to facilitate toner transfer, the toner is smudged to an extent that the image quality of the copy is unacceptable.